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Improving Mid-Infrared Imaging and Sensing

Scanning Electron Microscope image shows a few of the carefully designed shaped of the chalcogenide glass deposited on a clear substrate. The shapes, which the researchers call “meta-atoms,” determine how mid-infrared light is bent when passing through the material. (source: MIT)

Artificial optical materials could allow cheaper, flatter, more efficient detectors for night vision and other uses.

A new way of taking images in the mid-infrared part of the spectrum, developed by researchers at MIT and elsewhere, could enable a wide variety of applications, including thermal imaging, biomedical sensing, and free-space communication.

The mid-infrared (mid-IR) band of electromagnetic radiation is a particularly useful part of the spectrum; it can provide imaging in the dark, trace heat signatures, and provide sensitive detection of many biomolecular and chemical signals. But optical systems for this band of frequencies have been hard to make, and devices using them are highly specialized and expensive. Now, the researchers say they have found a highly efficient and mass-manufacturable approach to controlling and detecting these waves.

The new approach uses a flat, artificial material composed of nanostructured optical elements, instead of the usual thick, curved-glass lenses used in conventional optics. These elements provide on-demand electromagnetic responses and are made using techniques similar to those used for computer chips. “This kind of metasurface can be made using standard microfabrication techniques,” Gu says. “The manufacturing is scalable.”

These techniques allow the creation of metaoptical devices, which can manipulate light in more complex ways than what can be achieved using conventional bulk transparent materials, Gu says. The devices can also control polarization and other properties.

Mid-IR light is important in many fields. It contains the characteristic spectral bands of most types of molecules, and penetrates the atmosphere effectively, so it is key to detecting a wide range of substances such as in environmental monitoring, as well as for military and industrial applications, the researchers say. Since most ordinary optical materials used in the visible or near-infrared bands are totally opaque to these wavelengths, mid-IR sensors have been complex and expensive to make. So the new approach could open up entirely new potential applications, including in consumer sensing or imaging products, Gu says.

The research was funded by the Defense Advanced Research Projects Agency (DARPA), under the Extreme Optics and Imaging Program, and the National Natural Science Foundation of China.

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